"a" is a bad key for mark all read, since it is too easy to hit "a",

and mark all read is not undoable.  make mark all read "ctrl shift c"
like it was in 4.x.

mailnews/base/resources/content/mailWindowOverlay.xul

@@ -142,7 +142,7 @@ Rights Reserved.
 	<key id="key_selectAll"/>
 	
 	<key id="key_toggleRead" key="&markAsReadCmd.key;"     observes="cmd_markAsRead"/>
-	<key id="key_markAllRead" key="&markAllReadCmd.key;"     observes="cmd_markAllRead"/>
+	<key id="key_markAllRead" key="&markAllReadCmd.key;"     observes="cmd_markAllRead" modifiers="accel, shift"/>
 	<key id="key_nextMsg" key="&nextMsgCmd.key;"     observes="cmd_nextMsg"/>
 	<key id="key_nextUnreadMsg" key="&nextUnreadMsgCmd.key;"     observes="cmd_nextUnreadMsg"/>
 	<key id="key_nextUnreadThread" key="&nextUnreadThreadCmd.key;"     observes="cmd_nextUnreadThread"/>

mailnews/base/resources/locale/en-US/messenger.dtd

@@ -341,7 +341,7 @@ Rights Reserved.
 <!ENTITY markByDateCmd.accesskey "d">  
 <!ENTITY markAllReadCmd.label "All Read">  
 <!ENTITY markAllReadCmd.accesskey "a">  
-<!ENTITY markAllReadCmd.key "a">  
+<!ENTITY markAllReadCmd.key "c">  
 <!ENTITY markFlaggedCmd.label "Flag">  
 <!ENTITY markFlaggedCmd.accesskey "f">  
 <!ENTITY openMessageWindowCmd.label "Open Message">